1. Technical Field
The present invention relates to a catalyst deterioration diagnosis apparatus that diagnoses deterioration of the catalyst of an engine and, more particularly, a catalyst deterioration diagnosis apparatus that can diagnose even in a situation where the diagnosis condition is not satisfied within a time period required for executing the diagnosis.
2. Related Art
For example, in a gasoline engine for vehicles, after-treatment of HC, CO or NOx, such as contained in an exhaust gas, can be carried out by using a three way catalyst of which carrier may be made of alumina, for instance, with carrying noble metal, such as platinum, palladium, and rhodium.
Recently, due to needs for conforming to various government regulations, there has been demanded an on-board diagnosis technique that diagnoses the deterioration condition of a catalyst with high accuracy by relying only on devices available in the vehicle during its driving.
As a related art concerning the deterioration diagnosis of a catalyst, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2008-138556 discloses a technique for determining an abnormality using the estimated value of the O2 storage amount of a catalyst obtained when the output of an O2 sensor provided downstream of the catalyst is inverted from the lean side to the rich side.
In addition, JP-A No. 2008-121581 discloses a technique for calculating a deterioration diagnosis value based on the ratio between the output value variation integrated value of an air-fuel ratio sensor disposed upstream of a catalyst and the output value variation integrated value of an O2 sensor disposed downstream of the catalyst and, when the deterioration diagnosis value exceeds a threshold, the deterioration of the catalyst is determined.
The above deterioration diagnosis of a catalyst needs to be performed more frequently than a certain frequency (e.g., as per driving cycle) during use of a vehicle.
As to a diagnosis running condition to execute such a catalyst degradation diagnosis, there is a need for elapsing time that is required for the diagnosis (e.g., approximately several tens of seconds) while the vehicle speed, the amount of accelerator operation, and the like fall within a predetermined range under the condition without abrupt change.
However, depending on the characteristics of a market where the vehicle is introduced, the diagnosis running condition is more likely not to be satisfied before a period becomes long enough for executing the diagnosis, which diagnosis frequency turns out being not adequately secured. This may be caused by frequently occurring traffic congestion, the driver's frequent on-off operations of the accelerator pedal, and so on.